This invention relates generally to an improved method for lubrication of metal workpieces being formed at elevated working temperatures of at least 250.degree. C. and higher and more particularly to a novel lubricant combination for such method of metal forming which includes formation of a novel polymer lubricant in situ at the elevated working temperatures.
Various lubrication means are known whereby metal workpieces being formed at elevated temperatures with one or more die members, such as by forging or by extrusion, are provided with lubrication both prior to and during the metal forming operations. Both workpieces and die members are often heated to very elevated temperatures, particularly if ferrous metals are being formed, with the lubricant often being supplied in copious quantities to provide both lubrication and cooling of the die members. For example, U.S. Pat. No. 2,821,016 describes the hot forging of steel billets or slugs preheated at temperatures up to 2300.degree. F. and thereafter formed with movable and fixed die members being maintained with the liquid lubricant below 1000.degree. F. In doing so, the die members are flooded with a lubricating solution of colloidal graphite suspended in water containing a soluble oil. While such lubrication is reported to prevent "score+ marks on the forged product and die members, it has also been found that considerable cleaning of these articles is required to remove adherent carbon particles.
Other water-based lubricants have similarly been employed which are said to provide better lubrication means than achieved with "oil-base+ suspensions of graphite and still other particulates. For example, there is disclosed in U.S. Pat. No. 4,401,579 a lubricant composition employing fumaric acid salts as the primary lubricating and release agent for use in forging operations. As therein employed, such lubricant compositions can further include other suitable thickeners and polymethacrylates, polyvinyl alcohol, starch, gelatin, gum arabic and polysaccharides along with sufactants, wetting and dispersing agents. Suitable use of such lubrication means is further said to include other metal forming operations such as drawing, press forming, extrusion, wire drawing and other processes where workpiece temperatures reach at least about 800.degree. F. In a reported test the die members were preheated to 500.degree. F. with the die members being sprayed with the disclosed lubricant while low carbon steel billets heated to 2150.degree. F. were being forged therein. A different lubricant composition is disclosed in U.S. Pat. No. 4,765,917 for use in elevated temperature metal forming operations. This water-based lubricant is said to comprise about one percent to about forty percent by weight of a polycarboxylic acid salt reaction product, such as trimellitic acid and an alkali metal or an alkaline earth metal hydroxide such that the pH of the composition is about 6.5 to about 10 along with about 0.1 percent to about 12 percent by weight of a water dispersible thickening agent, and the balance water. Said water-based lubricant is said to further optionally include extreme pressure additives, performance enhancers and biocidal agents. Representative extreme pressure additives are said to include phosphate esters while listed performance enhancers include ammonium phosphate and alkali-metal polyphosphates. As therein employed, such lubricant composition is reported suitable in hot forging processes and other metal forming operations such as drawing, press forming, extrusion, wire drawing and like processes where workpiece temperatures generally reach at least about 1100.degree.-1300.degree. F. for aluminum pieces and 1300.degree. F.-2300.degree. F. (generally 1800.degree.-2000.degree. F.) for steel workpieces. The average die temperature is reported to be about 600.degree. F. with die temperatures varying from about 250.degree. F. to 900.degree. F. A reported test for hot drawing of steel artillery shell casings supplied such lubricant to the preheated punch or ram members over a time period varying between eight to eleven seconds with said time period said to be less than a twenty second spray period previously required with another prior art lubricant.
Various solid lubricants have also been employed as powders or particulates during the formation of metal workpieces at the aforementioned elevated work temperatures. A glass powder for such use is disclosed in U.S. Pat. No. 4,788,842 when forging ferrous alloy billets at working temperatures between about 800.degree. C. and 1200.degree. C. The solid lubricant is said to be removed from the finished article by sand blasting to produce a near metallic finish. In conducting the reported metal forming operation, such glass lubricant is applied as a coating to the preheated workpiece with the coated workpiece thereafter being forged. A different powdered lubricant is disclosed in U.S. Pat. No. 5,081,858 for the forging of hard to work metals such as stainless steel. The reported lubricant particles are electrically charged with high voltage for deposition on the preheated metal workpiece with the coated workpiece thereafter being found suitable for use in both cold and hot forging operations. Listed powdered lubricants include phosphoric acid, zinc calcium phosphate, metallic soap and oxalates.
It is also well known to lubricate various type mechanical systems operating at elevated temperatures with load bearing surfaces in dynamic physical contact, such as journal bearings, piston rings, gears, cams and the like. As the operating temperatures for these systems reach 300.degree. C. and higher so as to even approach the melting points of conventional metals now being employed, it has become essential that more effective lubrication be provided. A recently developed lubrication means for ceramic bearing surfaces is disclosed in U.S. Pat. No. 5,139,876. As therein described, formation of a tenacious lubricating film is achieved upon treating the uncoated ceramic bearing surfaces at elevated temperatures with activating metal ions to form a deposit of the activating metal ions on the ceramic surface and thereafter exposing the treated ceramic surface to a vaporized polymer-forming organic reactant at elevated temperatures whereby an adherent solid organic polymer lubricating film is produced on the treated surface. Bearing surfaces formed with crystalline ceramic materials such as silicon nitride and silicon carbide as well as vitreous ceramics such as fused quartz can be provided with a protective coating resistant to dynamic wear conditions up to at least 500.degree. C. and higher in this manner. In one embodiment, activated metal ions comprising a transition metal element is selected from the Periodic Table of Elements, to include iron and tin are initially deposited at temperatures of at least 300.degree. C. on the ceramic surface. Formation of a lubricating film on the treated ceramic surface is achieved with vapor deposition again being conducted at elevated temperatures of approximately 300.degree. C.-800.degree. C. with various polymer forming organic reactants such as petroleum hydrocarbon compounds, mineral oils, various synthetic lubricants and to further include tricresyl phosphate (TCP) and triphenyl phosphate. Similarly, a copending application Ser. No. 07/937,425 entitled "High Temperature Lubrication For Metal and Ceramic Bearings", filed Aug. 31, 1992, in the names of Edgar Earl Graham and Nelson H. Foster, now U.S. Pat. No. 5,351,786, describes lubrication means provided with still other novel organic polymer lubricants formed in situ. In said method of lubrication, both metal and ceramic bearing surfaces undergo reduction of the friction coefficient and surface wear when provided with a novel class of phosphazene polymer lubricants vapor-deposited during atmospheric bearing operation at elevated temperatures of at least 300.degree. C. During such operation the phosphazene starting compound becomes initially vaporized then polymerized in the vapor phase for subsequent deposition of the polymer product in lubricating amounts on at least one of the moving bearing surfaces. Suitable precursor reactants for such lubrication means include linear phosphazene, cyclophosphazene and cyclotetraphosphazene, including mixtures thereof, with a preferred reactant containing bis(4-fluorophenoxy)-tetrabis(3-trifluoromethylphenoxy) cyclotriphosphazene.
In a still more recently filed copending Ser. No. 08/109,949 application, filed Aug. 23, 1993 in the name of the present applicant and entitled "Elevated Temperature Metal Forming Lubrication", there is disclosed novel lubrication means when forming a preheated metal workpiece with a forming die at forming temperatures of at least 250.degree. C. The disclosed lubrication means polymerizes a vaporizable and polymerizable organic reactant selected from the group consisting of phosphate esters and phosphazene compounds to form a solid polymer lubricant in situ when contacting the forming die with the preheated workpiece. A water-based suspension containing such organic reactant can be applied to the shaping region of the forming die as well as further applied to the preheated metal workpiece in carrying out the disclosed lubrication means. Evaluation tests reported in FIG. 2 of said copending application demonstrate an average five percent reduction in applied pressure during the hot forging of a steel workpiece with the disclosed lubrication means as compared with employment of a conventional graphite lubricant. In the disclosed evaluation, a water-based graphite lubricant containing about sixteen percent by volume graphite was compared with an aqueous emulsion formed by adding ten percent by volume ethanol and 0.5% by volume Durad 620B to water. The Durad 620B precursor lubricant is a commercially available tertiary-butylphenyl phosphate ester supplied by FMC Corporation under said trade name.
It is one object of the present invention, therefore, to provide a still further improved method for lubrication of metal workpieces being formed at elevated working temperatures which is less subject to the cost and shortcomings now being experienced with conventional lubrication means.
It is another object of the present invention to provide novel lubrication employing polymer lubricants in combination with graphite particulates for use in various metal forming operations at elevated temperatures.
It is a still further object of the present invention to provide a novel method for lubrication of metal workpieces being formed at elevated temperatures which employs relatively low lubricant levels.
These and further objects of the present invention will become apparent upon considering the following detailed description of the present invention.